The invention relates to determining whether an electronic taximeter is in proper working order.
With conventional mechanical or electromechanical taximeters, it was not in general necessary to regularly test the taximeter to determine whether it was in proper working order. Any malfunctions in the interior of the device, on account of the mechanical transmissions between the input-speed rotary members and the fare indicating device, would become immediately apparent, because the indicating arrangement would simply cease to operate. However, with electronic taximeters the situation is quite different. With electronic taximeters, a very considerable period of time may pass before a malfunction of the electronic computing and indicating circuitry is perceived. This is because such electronic circuit malfunctions only seldom result in a stopping or perceivable inaccuracy in the progress of the counting action displayed by the taximeter indicating means.
With electronic computers, it is known to provide a built-in check of accuracy by using redundant electronic computer circuitry and performing all computations redundantly and in parallel. The results of the parallel, redundant computations are compared, and only if they are identical is there a readout of the results; otherwise a malfunction-indicating signal or the like is generated. A clear and very significant disadvantage of using this double-check expedient in an electronic taximeter is the expense of the redundant circuitry.
Another well-known error-detecting technique employed with electronic computer apparatuses is the use of a parity check. For example, if the computer handles 7-bit binary numbers, then there is added to each number on eighth or parity bit, which will be a "0" or a "1", depending upon the number of "0" and "1" bits in the 7-bit number. For example, the eighth or parity bit will be so selected that the total number of "0" bits in each such 8-bit number will be even. Special parity-checking auxiliary computer circuits are known for detecting errors by detecting loss or changes of parity in the binary-coded signals at various stages of the processing of the signals by the computer. The use of a parity check for error-detecting purposes in an electronic taximeter is at the present time prohibitively expensive, because of the considerable amount of additional circuitry required to effect the parity check.